(i) Manganese bronze

Composition: a copper-tin (bronze) base with a substantial addition of manganese - typically about 55-60% copper, up to ~40% zinc or tin with ~1-2% (or more) manganese (manganese bronzes are essentially high tensile free-cutting bronze). Properties: high strength, good hardness and castability, resistant to sea-water corrosion and to cavitation erosion, wear resistant.

Uses on board: propellers (fixed-pitch manganese bronze propeller blades) - chosen for strength, corrosion resistance, hardness and good casting and machining properties; also marine fittings/journals where strength and corrosion resistance are needed. Two examples: propeller blades/screw wheels, and shaft bushes/stuarts or sea-water-exposed fittings.

(ii) Cupro-nickel

Composition: copper with nickel, commonly 70/30, 90/10 or 55/45 copper/nickel (with small additions of iron/manganese for some). Properties: good strength, excellent corrosion resistance to sea water, resistant to biofouling and to pitting, good thermal conductivity, non-magnetic.

Uses on board: condenser/heater tubes and heat-exchanger tube bundles in sea-water systems - chosen for their excellent resistance to sea-water corrosion, biofouling and erosion so that cooling systems give long, trouble-free service; also piping for sea-water circuits.

Strength (ultimate/ proof/yield strength) is the stress at which the material fails or yields - the capacity to carry load without breaking/permanent deformation. Stiffness is the modulus of elasticity (Young's modulus) - the ratio of stress to strain in the elastic region, i.e. the resistance of the material to deflection under load; it is the same for all grades of steel (about 210 GPa).

Importance:

• Structural members (hull plating, frames, girders, casings): stiffness governs deflection and buckling resistance (a stiffer member resists bending/deflection and keeps the structure rigid without excessive movement), while strength governs the load the member can carry without yielding/fracture. On board, the deck girders, bulkhead stiffeners and the ship's side stringers are designed so they neither deflect unduly (stiffness) nor overstress/fracture (strength).
• Machinery components (shafts, crankshafts, frames, bedplates): strength determines the safe working load/fatigue life of a shaft or connecting rod; stiffness determines how much the member deflects under load, affecting alignment, clearances and vibration. For example, a shaft must be strong enough not to fracture under torque and stiff enough (and of correct section) not to deflect excessively, and bedplates must be rigid to hold the engine in alignment. Both are essential: high strength protects against failure, and stiffness controls deformation and hence the correct function and alignment.